Yes. But it would have color couplers. Kodachrome would not. So it would be a true B+W negative. Come to think of it, Kodachrome could be processed to be a B+W negative OR positive, in addition to a color positive.
The colour couplers in an E-6 film themselves are colourless. So you `practically´ would have a B&W negative. The same for unmasked C-41 films.
But keep in mind that all those films are not designed to yield `just´ a B&W negative.